Cytomegalovirus (CMV) is a major cause of morbidity and mortality in the immunocompromised host, especially in those having undergone solid organ or bone marrow transplants. A simple and sensitive method for early detection of CMV blood infection would be helpful for the medical management of these patients. Efforts have been made to determine the presence of CMV antigen in blood by detecting both the pp72 immediate early antigen of CMV in shell vial cultures and the pp65 lower matrix structural protein of CMV in polymorphonuclear leukocytes by using the direct immunofluorescent monoclonal antibody technique. Comparisons of the sensitivity, specificity, speed, and ease of determining CMV antigenemia indicate that direct detection of the pp65 antigen is more sensitive and more rapid, as well as an earlier predictor of CMV disease. Direct comparison of fluorescein- labeled with horseradish peroxidase-labeled monoclonal antibodies for detecting pp65 antigen have shown essentially equal sensitivity. As a result, we are currently using the immunoperoxidase method because its performance is faster and is not needed to read immunofluorescent smears. Further studies will compare results of shell vial culture and the antigenemia assay with a polymerase chain reaction assay use the immediate early gene region of CMV for primer recognition. Efforts to increase the sensitivity of the shell vial assay in detecting CMV, as well as other viruses, continue. Cells pretreated with iodo-deoxyuridine have been reported to enhance the replication of CMV in vitro. We will examine the effect of adding various concentrations of iodo-deoxyuridine, as well as other thymidine analogues, to our shell vial cell culture lines to determine if they enhance the sensitivity of the shell vial culture to detect CMV in clinical specimens.